Introduction: Dientamoeba fragilis (DF) is a commonly occurring intestinal protozoan that is considered a possible cause of infectious gastrointestinal disease in adults and children. DF has a particular high prevalence in children, and it is suspected that children present more symptoms in infection than adults. However, evidence of causality is lacking, treatment regimens are largely untested in controlled trials, and the most commonly used antibiotic against DF in Denmark, metronidazole, has never been tested against placebo.